The all-new Twitterrific 5 is kicking off this week’s must-have apps roundup, with a brand new look and a whole host of new features. It’s joined by the official YouTube client, which now supports iPad; an all-new Instacast; and a redesigned Gmail app.

When Twitter acquired and then ruined Tweetie, my favorite Twitter client for iOS, I turned to Twitterrific as a replacement. I used it for a long time, but I felt that other third-party clients — namely Tweetbot — were quicker to offer improvements and new features, and so I stopped using it. But now Twitterrific’s back and better than ever before.

Twitterrific looks nothing like the app it used to be. Thanks to its version 5 update, it sports a beautiful new user interface that’s very customizable, and lots of terrific new features. It’s nice and clean; it doesn’t get in your way too much, yet it’s well-designed and incredibly good-looking.

The best part about its UI is its customization options: You can change fonts and font sizes, switch between dark and light (black and white) themes, and change the line-spacing between text. A feature I really like is the ability to automatically switch to the dark theme at night — perfect for tweeting in bed.

Some of Twitterrific’s new features include pull-to-refresh (finally!) and other swipe gestures; discussion view; iCloud and Tweet Marker syncing; Pocket, Instapaper and Readability integration; threaded DMs view, and lots more.

YouTube was one of our must-have app picks when it was first released back in September, but it deserves another mention. Its latest update not only adds support for the iPhone 5’s larger display, but it also brings YouTube to the iPad. And it’s great; you’ll be thankful Apple pulled its own built-in YouTube app and inspired Google to build an official one.

Like Twitterrific, Instacast has been completely rebuilt and redesigned to make it better than ever. Here’s why you’ll want to upgrade:

This new version has been redesigned completely and now makes full use of iOS 6 technologies. It includes a completely redesigned iPad and iPhone app in one package and sports a new and robust syncing mechanism.

It is a new app, unfortunately, which means if you’re already an Instacast owner, you’ll have to pay for it again. But it’s well worth it. If you’re not a fan of Apple’s own Podcasts app, Instacast is by far the best third-party alternative.

Google didn’t just update its YouTube app this week; it has also overhauled Gmail as well. It’s got an awesome new look, and lots of new features, including multiple account support, search prediction as you type, infinite scrolling through your inbox, the ability to respond to Google Calendar invites, interactive Google+ support, and more.

What’s Your Favorite?

So that concludes our list of this week’s must-have iOS apps. If you’ve picked up something that you think we should have included, be sure to let us know about it in the comments.